1. Field of the Invention
The present invention relates to an internal combustion engine and, more particularly, to an exhaust gas recirculation system (EGR) for use with the internal combustion engine, in which an engine exhaust gas is partially recirculated into intake air to suppress excessive rise in the combustion temperature so that the emission of nitrogen oxides NO.sub.x can be reduced as much as possible.
2. Description of the Prior Art
In the aforementioned exhaust gas recirculation system, there is known to the prior art technical means by which the exhaust gas is recirculated at a flow rate proportional to the intake air flow rate into the intake air with a view to minimizing the emission of unburned noxious contents in the exhaust gas such as hydrocarbons HC or carbon monoxide CO and to effectively reducing the emission of the nitrogen oxides NO.sub.x. As the technical means thus known, there have been proposed the following three systems:
(1) Exhaust Back Pressure Control System; PA1 (2) Load Proportioning System; and PA1 (3) Throttle Valve Coacting System.
The system (1) notices the fact that the exhaust back pressure has a correlation with the flow rate of the intake air into the internal combustion engine, and controls the EGR vacuum by the back pressure. According to this system, if it is intended to effect a precise exhaust recirculation when in a low-load running operation of the engine, the exhaust back pressure has to be raised by throttling or reducing the diameter of an exhaust duct. However, if the exhaust duct is throttled while the internal combustion engine is run at a high speed, there arises a disadvantage that the exhaust resistance is increased so that a desired high-speed output cannot be generated.
On the other hand, the second system (2) makes use of the relationship between the intake air flow rate of the internal combustion engine and the venturi vacuum of a fixed venturi type carburetor. This invites a disadvantage that the system (2) cannot be applied to the engine which is equipped with such a variable venturi type carburetor as has its venturi vacuum varied even for an identical intake air flow rate if the effective area of the venturi is varied.
On the other hand, the third system (3) makes the throttle valve of a carburetor and an exhaust gas recirculation valve coactive and is known as a relatively reliable system. However, since the exhaust gas recirculation valve requiring a high operating force is connected to the throttle valve of a carburetor, the operations of opening and closing the throttle valve is dragged to exert more or less excessive load upon the throttling operation of a rider thereby to invite a disadvantage that the fatigue of the rider is increased after a long ride.